WSJ has a page 3 article on oil companies buying water rights in Colorado. Keep in mind west of the Mississippi, water rights are much like mineral rights, and are not necessarily part of land purchase. This is called Prior appropriation water rights.

The legal details vary from state to state; however, the general principle is that water rights are unconnected to land ownership, and can be sold or mortgaged like other property. The first person to use a quantity of water from a water source for a beneficial use has the right to continue to use that quantity of water for that purpose. Subsequent users can use the remaining water for their own beneficial purposes provided that they do not impinge on the rights of previous users.

Beneficial use is commonly defined as agricultural, industrial or household use. Ecological purposes, such as maintaining a natural body of water and the wildlife that depends on it, were not initially deemed as beneficial uses in some Western states but have been accepted in some jurisdictions. The extent to which private parties may own such rights varies among the states.[1]

Each water right has a yearly quantity and an appropriation date. Each year, the user with the earliest appropriation date (known as the "senior appropriator") may use up to their full allocation (provided the water source can supply it). Then the user with the next earliest appropriation date may use their full allocation and so on. In times of drought, users with junior appropriation dates might not receive their full allocation or even any water at all.

When a water right is sold, it retains its original appropriation date. Only the amount of water historically consumed can be transferred if a water right is sold. For example, if alfalfa is grown, using flood irrigation, the amount of the return flow may not be transferred, only the amount that would be necessary to irrigate the amount of alfalfa historically grown. If a water right is not used for a beneficial purpose for a period of time it may lapse under the doctrine of abandonment. Abandonment of a water right is rare, but occurred in Colorado in a case involving the South Fork of San Isabel Creek in Saguache County, Colorado.

Oil, Water Are Volatile Mix in West

DENVER -- Oil companies have gained control over billions of gallons of water from Western rivers in preparation for future efforts to extract oil from shale deposits under the Rocky Mountains, according to a new report by an environmental group that opposes such projects.

The group, Western Resource Advocates, used public records to conclude that energy companies are collectively entitled to divert more than 6.5 billion gallons of water a day during peak river flows. The companies also hold rights to store, in dozens of reservoirs, 1.7 million acre feet of water, enough to supply metro Denver for six years.

Industry representatives said they have substantial holdings of water rights for future use in producing oil from shale, though they could not confirm the precise numbers in the report.

Before any move into full-scale oil shale production, the energy industry plans a close study of water issues, including the impact its operations would have on ranchers, farmers and communities that all rely on the same limited sources of water, said Richard Ranger, a senior policy adviser for the American Petroleum Institute. "It's among the most important questions to be examined," he said.

Why is water important to oil companies?

But if the price of oil rebounds, the potential payoff is big: the federal government estimates 800 billion barrels of oil, triple the known reserves of Saudi Arabia, lie under the Rocky Mountain West.

For now, the energy companies are not using most of the water they've claimed; they're leasing some of it to other users, most often farmers. But they are stocking up on water rights to be sure they won't be caught short.

Associated Press

Colorado River water supports 30 million people and dozens of uses, including power generation, above, at the Glen Canyon Dam in Arizona.

We're picking up properties as they become available or look strategic," said Tracy Boyd, a spokesman for Royal Dutch Shell PLC. Shell does not expect to need large quantities of water for at least 15 years, he said, and by then it may have developed less water-intensive ways to extract oil, perhaps using wind power.

Exxon Mobil Corp., too, said new technologies might reduce future water needs. "We continue to be a careful steward of this precious resource and a considerate neighbor in dry years," said Patrick McGinn, a spokesman.